Josh Ryan addresses the question in this manuscript, to be published in Electoral Studies.  Here’s the abstract:

Among the reasons for the historic nature of the 2008 Democratic primary race was the attention paid to the “superdelegates”. The competitiveness of the primary and the important role the superdelegates played has led to calls for reform. This paper develops a formal model that explains why superdelegates selected one candidate over the other and why some superdelegates committed early in the primary season while others waited. Hypotheses are tested using an original dataset collected during the 2008 Democratic primary. The results suggest that although some superdelegates made their decision based on personal, idiosyncratic factors, for many superdelegates, Democratic voters played the most important role in their commitment process.